The present invention relates in general to bolometers, i.e. temperature measuring devices utilizing a temperature dependent resistance, and in particular to an ultrafast superconducting bolometer.
The measurement of temperature is one of the chief tools for the measurement of a host of other physical quantities. Besides the usual measurement of ambient temperature as would be performed by a thermometer, temperature measuring devices can be used in conjunction with a heat-isolated chamber in which some other mechanism occurs which releases or absorbs heat. If the specific heat of the chamber is controlled, the resulting temperature changes measure the energetics of the mechanism occuring within the chamber. Examples of such uses are the heat of reaction in a chemical reaction, phase changes indicated by changes in specific heat, absorbed microwave power and radiance of an infrared source.
Some related applications do not require thermal equilibrium between the thermal sensor and the heat source. In such applications as photon detectors and phonon detectors, a packet of waves is absorbed by the detector or a secondary absorber in contact with the detector and the wave energy is converted into an equivalent quantity of heat. The detector's temperature rise is related to the energy of the wave packet. In this use the incident waves are not continuous but form a pulse so that the wave packet is detected as a temperature pulse if the detector is in good thermal contact to a heat sink. Such use requires high responsivity per unit of incident energy because the heat content per pulse is low. Furthermore, if such a photon or phonon detector is being used where the information rate is high, e.g. in a submillimeter wave communication network or a phonon signal processor, the detector must have very fast response. R. J. von Gutfeld and A. H. Nethercot, Jr. in Physical Review Letters, vol. 12, pages 641-644, 1966 have described the propagation of phonons or a heat pulse and their detection by bolometers which are resistive components inserted in an electrical circuit the resistance of which is strongly temperature dependent. Some of the temperature measuring techniques other than bolometers in the above fields of measurement include thermocouples and thermistors. Thermocouples measure temperature by sensing the voltage present across a junction of dissimilar metals, which voltage varies with temperature. Thermistors are temperature dependent semiconductive resistors which are quite sensitive between -100.degree. and 300.degree. C. However such temperature measuring devices have a rather limited temporal response because of their significant bulk. For example, fast thermocouples have temporal responses measured in milliseconds.
The time response of bolometers can be improved if they are made in the form of an electrically conducting thin film deposited upon an substrate that is thermally conducting but electrically insulating. For pulsed wave detectors, such construction has the potential of providing a good heat sink through the substrate to more elaborate cooling means so that a wave pulse is converted into a temperature pulse in the bolometer which in turn is detected as a current or voltage pulse by the detecting electrical network. However fast response to heat pulses implies that heat flows quickly from the active volume of the bolometer so that relatively little energy is available to drive the detector. As a result high responsivity in the bolometer is required.
The fast response of metallic thin film bolometers relies on the small specific heat associated with a thin film such that the heat generated in the film can be quickly drawn off into the substrate. The thinner the metal film, the more quickly can the heat be dissipated. Von Gutfeld and Nethercot in another article in Physical Review, vol. 37, pages 3767-3771, 1966 describe the use of both normal and superconducting indium films on sapphire substrates with thermal response times of down to 2 ns. The temporal response is limited by the thinness at which metallic thin films can be grown on a dissimilar insulating substrate and have acceptable conduction or superconducting characteristics. It appears that 50 to 100 nm is a lower limit for the thickness of self-supporting metal films with the best response times being the 2 ns previously quoted.
The responsivity of superconducting bolometers can be enhanced by operating them near their transition temperatures where the temperature dependence of the bolometers resistance is relative large. However these usually quite low transition temperatures require complex equipment and restrict the applications for which superconducting bolometers can be used. For example, film bolometers have been reported with ambient temperatures of 3.8.degree. K. for superconducting In-Sn, of 8.degree. K. for superconducting Pb-Bi and of 1.5.degree. K. for superconducting granular aluminum.
A general problem with thin-film superconducting bolometers is that they tend to be made of exotic materials with poor mechanical characteristics and which are prone to chemically react and hence change their superconducting characteristics. Their thinness causes minor scratches to produce major changes in performance.